When they are hatched under a broody they are usually about 6/7 weeks old before Mum decides they are feathered up enough to be left, so I would say they need a little heat at least at night for a couple of weeks more, and then it will depend on the weather. If there were more of them to huddle together it would not be as crucial, but three in a large cage will not provide them with much body heat from each other. As far as them mingling with the others, opinion will vary greatly on this, but as they have no Mum to 'stick up' for them, I would always do it in short sharp bursts, and always supervised. Personally, I would wait until they are well feathered (6/8 weeks) and then for only half an hour at a time to start with. By then the weather should have warmed up and you can take your G & T and sit in the sun with them so you can keep an eye on proceedings, as I'm sure you are aware of how bitchy older hens can be ! ;)